Sewage sludge is typically processed by municipalities into a product called “cake”. This is the result of incubation with specialized bacteria for a prescribed length of time, then processing with a polymer prior to squeezing as much water out of the mixture as possible. The cake so formed is then disposed of in a variety of ways, all of which have problems. The most common means of disposal has been incineration, but studies are revealing that the metals, some of which are toxic, present in the sewage are accumulating in the soils around incinerators.
The second most prevalent means of disposal is the spreading of the partially processed sewage on agricultural, park, and other land. The sewage still has a significant number of pathogens present, however, which limits the type of crop grown and the time in which the harvest can safely be made. Sewage which is processed more than that used directly on the land is sometimes used for other types of fertilizer. The extent to which this can be done depends on the thoroughness of the processing, and the survival of some of the pathogens.
Some sewage sludge, (the cake), is sometimes landfilled. The persistent bacterial activity causes more methane to form than other waste. The problem of pathogen- and metal-leaching into the groundwater remains a concern.
All in all, the sewage remains a problem which has not resulted in a good solution. Sewage is a good source of organic feedstock for selected processes, and is underutilized.